Filters and the like that use a bed of filter media to remove solids from liquids are well known. Such systems typically include a liquid distribution system that collects liquid after it is filtered in one flow direction and distributes clean liquid through the filter media in a reverse flow direction to effect a cleaning process known as backwashing.
Water filtration systems of the gravity type are commonly employed for filtering high volumetric flow rates of water, e.g., in municipal and industrial water treatment and waste water treatment plants. The filtration systems generally comprise one or more filters, each employing a bed of granular filter media for filtering a liquid as it seeps downward through the filter bed.
Each filtration system generally comprises an open filter basin having a floor and vertical walls surrounding the floor and an underdrain positioned directly above the floor. The underdrain defines a perforated false bottom in the basin for supporting the filter bed and to provide a system of fluid passageways for removing the filtered water from the bottom of the filter basin.
The filter bed is generally several feet deep and comprises successive layers of gravel and sand of graduated sizes. A layer of relatively coarse gravel is provided at the bottom of the filter bed, lying on the upper surface of the underdrain. This layer of support gravel is provided in progressively finer sizes toward the top of the filter bed so that media is not lost by penetrating the layer of support gravel.
On top of the support gravel would rest the bed of filter media, which can contain one or several types of media material. Typically, the bottom-most layer consists of finer media having higher specific gravity relative to the uppermost layer, which consists of coarser media having a lower specific gravity.
During operation of the filtration system, the influent, i.e., unfiltered water, is directed into the filter basin to a depth of several feet above the upper layer of filter media. The influent is allowed to flow downward though the filter bed. During this process, the suspended materials in the unfiltered water become trapped in the filter media. The water ultimately reaches the bottom of the filter bed and passes through the perforations in the underdrain system. The water is then collected in a system of fluid passageways within the underdrain system and is carried out of the filter basin through a suitable conduit or flume.
After the filtration system is operated for an extent of time, the efficiency of the system decreases and it becomes necessary to wash the filter bed to remove the foreign materials trapped therein. Washing of the filter media is accomplished by utilizing a backwashing process. The backwashing process involves pumping pressurized water and/or air through the flume in a reverse direction into the system of fluid passageways in the underdrain system, upward through the perforations in the underdrain, and into the overlying filter bed.
When fluidization has been achieved, the wash water flowing upward through the filter bed carries the foreign articles upward from the filter bed. Fluidization is achieved when the bed of filter media reaches complete expansion. Complete expansion occurs when the rate of water pumped into the bed has a velocity that is just strong enough to keep the filter media suspended. Typically, fluidization is not achieved until the bed expands to at least approximately 30%, maybe not until 50% expansion, and sometimes up to 100% expansion is required. The wash water and the foreign materials entrained or suspended therein are then collected at the top of the filter basin in a waste trough and carried away.
The backwashing process must be performed under carefully controlled conditions so as to achieve effective cleaning of the filter bed while avoiding disruption or damage. When water is used in the backwashing procedure, it is first pumped into the filter bed at a relatively slow rate and increased until fluidization is achieved. At this particular flow rate, the velocity of the water and its density and viscosity will fluidize or lift the particles, thereby expanding the bed of filter media so as to allow the flow of water to easily carry away the foreign particles. This fluidization rate is then kept constant for approximately 5 to 7 minutes while backwashing is performed. The filter bed is then ready for filtering operation and the cycle continues for the life of the bed.
To maximize the potential life of a filter bed, it must be washed properly. Proper washing entails backwashing at or very near the fluidization rate. Underfluidization, backwashing at a rate below the fluidization rate, results in ineffective cleaning because without complete bed expansion for fluidization, some foreign particles remain trapped among the filter media no matter how long one performs the backwashing process. Underfluidization may also result in a non-uniform distribution of wash water throughout the filter bed, thus not effecting a complete washing of the entire filter bed. If the wash water distribution is uneven so that dead spots occur at certain locations within the filter bed, then those portions of the filter bed will not be properly cleansed, thereby reducing the efficiency of the filter.
Overfluidization, backwashing at a rate above the fluidization rate, results in potential loss of filter media because the force of the water will simply cause the filter media to become entrained in the wash water along with the removed materials and carried away as waste. "Blow holes," in which explosive bursts of wash water open channels in the filter media at the initiation of the backwashing cycle, must also be avoided. During filtration, these blow holes allow influent to pass through the filter media without being filtered and allow finely-sized filter media to be carried away with the effluent, i.e., the filtered water.
It is known in the art that there is no head loss across a bed of filter media, i.e., vertically across, when the bed is fluidized. That is, the differential pressure between the pressure at the top of the layer of support gravel and the pressure at a certain distance above the upper-most layer of media approaches a constant value as the bed approaches 100% fluidization. Thus, if one is monitoring the differential pressure across the bed of filter media while increasing the rate of water flow into the filter bed, one can determine when fluidization occurs and then begin backwashing at that determined rate.
The prior art apparatus used to measure the differential pressure across the entire filter bed comprises a pipe and a transmitter for each point to be measured. To install one of these prior art transmitters, one has to pierce the wall of the filter basin at the appropriate height level of measurement, placing the pipe perpendicular to the direction of the flow and directing the transmitter, placed on the end of the pipe, against the direction of flow. Typically, these transmitters are simple manometers. Installing the prior art devices is an unnecessarily burdensome endeavor requiring significant man-hours and modifications to a filter basin. Moreover, the prior art transmitter can not be installed on a functioning filter bed or one with a bed of filter media already in place. Thus, it is desirable to provide a user-friendly device for measuring the differential pressure across a filter bed that can be installed in a retro-fit manner.
As a result of the rather involved task of utilizing this prior art technique, it was seldom, if ever, used. Thus, to determine the proper fluidization rate for a given filter bed, the prior art technique is to first measure the temperature of the water in the filter bed. From this temperature measurement, one can approximate the density and viscosity of the water at that temperature using generally available references. From there, and knowing the average size of the filter media in the bed, one can use an American Water Works Standards reference to find the flow rate needed to expand the filter bed, i.e., the fluidization rate.
Unsurprisingly, the above prior art technique is highly unreliable in determining the fluidization rate for a given filter bed. Thus, it is also desirable to provide a device for measuring the differential pressure across a filter bed so as to monitor a filter bed and achieve efficient control of filtration and backwashing procedures.